Image sensing devices that capture monochrome or color images by changes in the electrical properties of photo-sensing pixels on integrated circuit dies or chips have provided an alternative to traditional film technology. Known types of the sensors include field effect transistor (FET) or diode devices, fabricated with complementary metal oxide semiconductor (CMOS) or charge couple device (CCD) technology.
CMOS and CCD image sensors each have advantages and disadvantages. CMOS technology offers ease of interfacing with other CMOS based hardware and can reduce power drain on portable devices such as digital cameras, video phones, PDAs and other appliances and especially battery powered units. CCD equipment is an older technology and is in many cases more easily fabricated with high pixel densities. Both of these and other sensors typically require blocking of infrared (IR) light energy from the imaging light that contains the desired visible image. Also, it may be desirable to filter out ultra violet (UV) energy from the imaging light.
This is because the most common semiconductor-based image sensing devices such as the above are silicon-based and respond not only to visible light (approximately 380 to 780 nanometers), but also to infrared light in the range of approximately 780 to 1100 nanometers. Quality color capture without a blocking filter for these wavelengths is virtually impossible because the IR typically swamps the sensor performance and thereby corrupts the output levels. Similarly monochrome capture relies on preserving the luminance in the sensed visible light, which is difficult without IR filtering.
There are two types of common IR filters, which filter out energy in the infrared region of the electromagnetic spectrum. The first kind is blue colored glass such as made by ionic coloration. Such colored glass is relatively expensive, approximately 20 times more expensive than clear glass. Another type of filter element uses a clear piece of glass that has a thin film coating on one surface to cause cancellation by interference of those incoming wavelengths such as IR that are outside the visible spectrum. This type of filter uses multilayer thin-film stacks with designed thicknesses, usually in the nanometer (nm)range, to pass and reflect a selected wavelength. To filter IR, the cutoff wavelength is approximately 630 nm, with transmission dropping to approximately 50% at 650 nm, and dropping to a few percent at 680 nm. One problem with this approach, however, is that the thin film coating is usually optimized for particular wavelengths and for light rays normal to the thin film surface. Therefore, the infrared light rays near the edges of the pixel array, which usually exit the field lens at an angle, are less effectively filtered. This edge effect usually shifts the 50% transmission wavelength from 650 nm to less than 630 nm, thus shifting the color of the imaged light toward the red part of the spectrum. This may cause a visual artifact at the edges of the image known as a color discontinuity. In some cases, image processing can be used to remove the color shift, but at added cost and complexity.
While effort has been made to provide IR and UV blocking without sacrificing the image integrity or adding excessive cost and complexity to the image sensing optics, the approaches thus far fall short in one or more respects. For example, certain digital imaging systems incorporate a separate filter, typically glass or plastic, as part of the optical train, i.e., somewhere in the optical system either in front of or between the field lens and the sensor package. The field lens or fieldlens assembly is usually separated from the sensor by air or other fill gas and this open chamber may be sealed off in a package, sometimes called a shell case. Mounting of the filter in the case chamber may result in dust and other contaminants lodging on the sensor pixels and obstructing the image rays. Another known approach places an IR filter element over or at a window of the sensor package above the sensor die.
Such a placement is shown by way of example in FIGS. 1A and 1B marked prior art, where a package or case 15 contains an image sensor die 17. A window on the case is covered by a filter element 25, which may be made of coated glass or a plastic material containing a dye for absorbing IR wavelengths.
The separate selective IR and/or UV filter element 25 adds an additional component to the total system count, i.e., piece-part count, and adds to the complexity and manufacturing cost of the imaging system. Further, depending on where the selective filter 25 is placed, there may be an increase in the system size that diversely affects cost of performance. Moreover, often the separate filter is either thin film coated on or made by doped glass and then cut into small pieces and mounted. The resulting dust and particles from these operations may contaminate the camera sensor and block active sensor pixels.
Sensor pixels that receive the filtered light are shown for example on a prior-art sensor die 17′ and a microlens 30 in FIG. 1B. The die 17′ is one of many identical dies that are typically manufactured simultaneously in a semiconductor wafer. The dies are often formed layer by layer, and result in a grid or array of photodetecting sites 20. Other integrated circuit elements (not shown), such as transistors, which cooperate with the photodetecting sites provide an image signal as well known in this technology. The photodetecting elements and the other circuit elements are interconnected using one or more metal layers (not shown). Color filter material (not shown) may be then deposited into wells across the entire wafer, usually above a passivation layer (not shown). This material is repeatedly deposited and patterned so that several different color filters are disposed to direct light of a specific color onto a receptive sensor element. To improve the efficiency with which photosensing sites 20 respond to incident light, a microlens structure 30 is attached to or formed on die 17′ and focuses the incident light onto photodetecting sites 20.
Sensor chip or die 17′ may be made using CMOS or CCD technology. For many years, CCDs have dominated the market in terms of speed, sensitivity, reliability, packaging and price. However CMOS devices offer the advantages noted above and effort is being made in CMOS technology to meet or exceed all critical price and performance characteristics of CCDs, and to deliver advantages to product developers that CCDs do not offer. One design factor in this effort is to use an array format incorporating the microlens structure described above. As CMOS based sensors compete with CCD devices, the provision of microlens arrays over the photosensing sites becomes important. Microlenses allow the use of smaller transistor sites which in turn afford increased sensor density and improved fill factor.
The microlens structure 30, being an array of hemispherical, cylindrical, or other shaped lens elements, is often made from an optically clear polymer resin material such as a durable acrylic that is molded or cast into the shape of the desired lens array and adhesively attached to the sensor die. In other fabrications the resin is spun onto the entire wafer and processed by photoresist and etching into the microlens shapes. The resulting microlens layer may have a thickness of 1-3 μm for example. Suitable materials for a microlens structure include those materials such as acrylic plastics and thermally or UV-light curable epoxy that have high transmissivity (greater than 90 percent) across the visible spectrum of light (380-780 nm), and are resistant to aging effects (e.g., oxidation, decomposition), environmental effects (e.g., moisture uptake, heat resistance), and physical effects (e.g., stress, deformation).
But as discussed above, a problem with the above-described imaging system is that the IR filter element 25 may increase the size and part count, and thus the cost, of the system, and may introduce contamination that reduces the performance of the system. Furthermore, where the filter element 25 is coated with a thin film, the filter element may introduce a color discontinuity into the captured image.